Many countries in the world have large deposits of oil sands, including the United States, Russia, and various countries in the Middle East. However, the world's largest deposits occur in Canada and Venezuela. Oil sands are a type of unconventional petroleum deposit. The sands contain naturally occurring mixtures of sand, clay, water, and a dense and extremely viscous form of petroleum technically referred to as “bitumen,” but which may also be called heavy oil or tar.
The crude bitumen contained in the Canadian oil sands is described as existing in the semi-solid or solid phase in natural deposits. Bitumen is a thick, sticky form of crude oil, so heavy and viscous (thick) that it will not flow unless heated or diluted with lighter hydrocarbons. The viscosity of bitumen in a native reservoir is high. Often times, it can be in excess of 1,000,000 cP. Regardless of the actual viscosity, bitumen in a reservoir does not flow without being stimulated by methods such as the addition of solvent and/or heat. At room temperature, it is much like cold molasses.
Due to their high viscosity, these heavy oils are hard to mobilize, and they generally must be made to flow in order to produce and transport them. One common way to heat bitumen is by injecting steam into the reservoir. The quality of the injected fluid is very important to transferring heat to the reservoir to allow bitumen to be mobilized. Quality in this case is defined as percentage of the injected fluid in the gas phase. The target fluid quality is near 100% vapor, however, injected fluid in parts of the well can have a quality below 50 percent (more than 50% liquid) due to heat loss along the wellbore.
Lesser quality injection fluid has a lower latent heat to transfer to the reservoir, causing inefficiencies and difficulties in oil sands operations, such as irregular shaped steam chambers, control issues, and reduction in mobilized fluids. In steam assisted gravity drainage, lower quality steam is generally observed at the end of the well due to steam condensing as it goes farther into the well and loses heat. This limits the practical length of lateral wells in steam assisted gravity drainage project to less than 1,000 meters.
One theoretical way of heating the wellbore, reducing latent heat losses and allowing longer wells might be to apply electromagnetic energy to the wellbore and/or fluid therein. Electromagnetic waves can certainly heat various materials, and microwave energy is often used to heat water. However, no one has used RF waves in this capacity before, although RF has been used in other down-hole applications.
U.S. Pat. No. 2,757,738, for example, is a very early publication disclosing a method for heating subsurface oil reservoir bearing strata by radio frequency electromagnetic energy, where the RF electromagnetic energy is generated by a radiator within a vertical well bore. The antennas of this method are not immersed in the ore for extended distance because the well bores are vertically drilled. Additionally, the vertically drilled well bores have inherent limitations on separating the charges between horizontal earth strata.
U.S. Pat. No. 3,522,848 discloses radiation generating equipment for amplifying the oil production in a natural reservoir. In essence radio frequency electromagnetic waves are used to heat the dry exhaust gas (comprising CO2 and nitrogen) of an internal combustion engine, and the heated gas is subsequently used to heat the reservoir to reduce the viscosity of the hydrocarbons contained in the natural reservoir.
U.S. Pat. No. 4,638,863 discloses a method for stimulating the production of oil by using microwave to heat a non-hydrocarbonaceous fluid (such as brine) surrounding a well bore, and the heated non-hydrocarbonaceous fluid will in turn heat the hydrocarbonaceous fluid in the same formation.
U.S. Pat. No. 5,236,039 provides a system for extracting oil from a hydrocarbon bearing layer by implementing RF conductive electrodes in the hydrocarbon layer, the RF conductive electrodes having a length related to the RF signal. The spacing between each RF conductive electrodes and the length of such electrodes are calculated so as to maximize the heating effect according to the frequency of the RF signal. However, the inventors' experiences indicate that standing wave patterns do not form in dissipative media, such as hydrocarbon ores, because the energy will be dissipated as heat long before significant phase shift occurs in the propagation of electromagnetic energies. Thus, this method is of limited use.
U.S. Pat. No. 7,091,460 discloses a method for heating a hydrocarbonaceous material by a radio frequency waveform applied at a predetermined frequency range, followed by measuring an effective load impedance initially dependent upon the impedance of the hydrocarbonaceous material, which is compared and matched with an output impedance of a RF signal generating unit.
US20070289736 discloses a method of in situ heating of hydrocarbons by using a directional antenna to radiate microwave energy to reduce the viscosity of the hydrocarbon. The method preferably applies sufficient energy to create fractures in the rock in the target formation, so as to increase the permeability for hydrocarbons to flow through the rough and be produced. However, directional antennas are not practical at the frequencies required for useful penetration, because the instantaneous half depth of penetration may be too short. For example a 2450 MHz electromagnetic energy in rich Athabasca oil sand having conductivity of 0.002 mhos/meter is 9 inches. Thus, this method is also of limited use.
WO2010107726 discloses a process for enhancing the recovery of heavy hydrocarbons from a hydrocarbon formation. Microwave generating devices are provided in horizontal wells in the formation, and a microwave energy field is created by the microwave generating devices, so that the viscosity of the hydrocarbons within the microwave energy field can be reduced and more readily produced. Electronic waves must be generated for this method to work, limited its usefulness.
However, none of the abovementioned literature discloses a method or system that addresses the issue of loss of latent heat of the steam during SAGD start-up operation, which may allow the extension of lateral well and reduce the number of wells being drilled. Thus, what is needed in the art is a method of efficiently heating the wellbore, such that longer wells can efficiently be used without latent heat losses.